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Dirty Pretty Snowballs

Dirty Pretty Snowballs

By Shamini Bundell

Everyone thinks the little children playing in the clean white snow on a winter’s morning are just having some innocent fun. What we are only now beginning to realise is that they are actually engaging in germ warfare. We may have to revise the phrase ‘pure as the driven snow’ with the discovery that each snowball is full of bacteria.

It’s not just that children are covering each other in pathogens – across the world bacteria are falling from the sky in raindrops and snowflakes. It seems that a number of microbes have a rather cunning way of spreading themselves around: getting blown into the atmosphere and then falling onto new hosts as precipitation.


Pure water in the atmosphere will not freeze until temperatures drop way below freezing. This is because the vapour requires a particle around which to condense and crystallise; these particles are known as nucleators. Bacteria, acting as nucleators, are able to condense cloud vapour into ice crystals at just a few degrees below freezing, thereby catalysing their own return to Earth. While it has long been known that bacteria and other microorganisms blow around in the atmosphere, scientists have never really considered their importance in affecting weather.

Previous studies that have tried to understand what makes ice crystals stick together have filtered out only the smallest nucleators - inanimate particles such as dust and soot. But what American scientists Brent Christner and colleagues have found is that in some places up to 85% of the nucleating agents are bacteria. “Bacteria are by far the most active ice nuclei in nature," says Christner, assistant professor of biological sciences at Louisiana State University. Bacteria have been found in precipitation across the globe, even in Antarctica.

 Now before you start locking your children inside the house and blocking up the chimneys, I should assure you that these bacteria are perfectly harmless – unless you happen to be a tomato plant. The most common ice nucleator, Pseudomonas syringae, affects only plants such as tomatoes and beans. The focus has previously been on eliminating it, but now that we understand a whole new stage in its life cycle, we must consider the important effect these bacteria may have on rainfall.

The ‘bio-precipitation cycle’ is only just beginning to be recognised and is going to be important in the future for understanding humans’ effect on and control over the climate – both deliberate and accidental.

If you thought that was good, check out Shamini's other articles.

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Image: Alaina Cherup

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19 Dec 2009
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